1. Field of the Invention
The invention relates to personal computers, and more particularly to loading of the operating system of the personal computer by devices external to the computer.
2. Description of the Related Art
While personal computers are becoming quite reliable and their operating systems are becoming quite stable and powerful, errors still do occur and at times it is desireable to load other operating systems onto the personal computer. In addition, diskless computers, generally referred to as workstations, are becoming quite prevalent on local area networks for security reasons.
Conventionally the initialization sequence of loading of the operating system, commonly referred to as booting of a personal computer is as follows. The personal computer is turned on and goes through certain initializations, such as an initial random access memory (RAM) check, initializing the interrupt controllers, direct memory access (DMA) controllers and other devices, and clearing and initializing the video display units and various input/output (I/O) devices. After the memory has been fully tested and initialization completed, the system attempts to boot itself from one of the magnetic media, generally either a floppy disk or a hard disk. This is done because usually the operating system is quite variable and large and therefore it is not desireable that it be located in the relatively expensive and limited amount of read only memory (ROM) present on a computer system. By placing the operating system on floppy disk or hard disk it can be much larger and can be much more variable. Thus it can be frequently changed without the difficulty of opening up the computer and replacing integrated circuits. Conventionally, personal computers first tried to boot from the floppy drive. This procedure developed when floppy drives were all that were available in a system and has continued as a means for allowing a computer to be booted if there is a failure to the hard disk drive or if alternative code which does not utilize the operating system present on the hard disk is to be used. If no floppy boot was possible, then control proceeds to try and boot from the hard disk. If that does not occur, errors are indicated and the computer did not boot. The term booting developed because a limited number of program instructions are first loaded from a known location on the disk. The first instructions included instructions to allow further instructions to be loaded from the disk. Because the programs are initially small and get larger, the process was referred to as booting up in a reference to pulling oneself up by one's bootstraps.
However, as previously stated, errors can occur in this process such that neither the floppy nor the hard disk drives are available for booting, either due to equipment malfunction or their complete absence in the computer if it is a diskless work station. Therefore, for diagnostic purposes or network purposes it is desireable to have a personal computer which can be started and then booted up even if both the floppy and disk drives are inactive or not present. Then various diagnostic tests can be performed or the network operating system loaded and executed.